Human immunodeficiency virus (HIV) is the etiologic agent of acquired immunodeficiency syndrome (AIDS). HIV infection leads to depletion of CD4+T lymphocytes. AIDS is characterized by various pathological conditions, including immune incompetence, opportunistic infections, neurological dysfunctions, and neoplastic growth.
The evolutionary success of primate lentiviruses is evident in their high prevalence in old-world primates, and their capacity to spread to new host species, frequently leading to the emergence of zoonotic disease. Their capacity to establish persistent infection in individual hosts also requires rapid and extensive viral adaptation which allows viral escape from humoral and cell-mediated immune responses. Viral adaptation to antiviral drugs underlies drug resistance which limits therapy for many patients. High rates of mutation contribute to high adaptive capacity of primate lentiviruses, and arise from the low fidelity of reverse transcriptase and RNA transcriptase II that are involved in replication of the viral genome. The mutagenic action of cytidine deaminases, such as APOBEC-3G, leads to hypermutation that is lethal. The role of this interaction in viral evolution is limited by its lethal phenotype, which restricts spread of possibly adaptive mutants, and the action of HIV-1 virion infectivity factor (Vif) which suppresses APOBEC-3G action by targeting the protein for ubiquitination.
One of the striking traits of HIV-1 genetic variation is G to A hypermutation, that is substitution of G residues for A in the proviral DNA as compared to the genomic RNA strand. G to A transition is strongly associated with the dinucleotide context decreasing in the order GA>GG>GT>GC. All parts of the retroviral genome can be susceptible to this process. Hypermutation occurs during reverse transcription when the minus DNA strand is synthesized.
Several recent reports have shown that a principal mechanism of G to A hypermutation is achieved by direct cytidine deamination of the retroviral minus stand cDNA during the reverse transcription. Cytidine residues are deaminated to uridine residues and this gives rise to G to A changes on the plus cDNA strand. Mutation within a GG context seems to be the consequence of APOBEC3G (h3G) action. APOBEC3G inhibits the replication of wide range retroviruses by mediating the lethal deamination of Cs to Us as the retroviral RNA is copied into DNA. APOBEC3G belongs to a family of ten cytidine deaminase genes that in humans includes APOBEC1, APOBEC2, AID and APOBEC3A to 3G. APOBEC3F (h3F) and APOBEC3B (h3B) create lethal G to A changes in the newly synthesized virus. These mutations may result in a virus that is degraded or nonfunctional. Antiretroviral activity of h3G, h3F, and h3B requires their encapsidation into assembling virions. However, HIV-1 encodes a protein Vif (virion infectivity factor) that reduces their incorporation into virions, possibly by promoting their degradation via the ubiquitin-proteasome pathway. A single amino acid change at residue 128 in the N terminal region of APOBEC30 determines the specificity for Vif susceptibility. The protein domain coded by residues 104-156 in the N terminal region of h3G is required for its incorporation into the virus.
Hypermutants have been identified in a single cycle of reverse transcription with wild type viruses. In clinical isolates, hypermutation occurs preferentially within a GA context. h3F and h3B have a GA context preference but the activity of h3F is susceptible to HIV-1 Vif and the expression of h3B is absent in PBMC susceptible to HIV-1 infection. Detection of HIV-1 hypermutated sequences in at least 43% of patients suggests that hypermutation may happen in a systematic way in HIV-1-infected individuals and may be associated with a viral mechanism to prevail in a hostile environment.
Several drugs, have been approved for treatment of AIDS, including azidovudine (AZT), didanosine (dideoxyinosine, ddI), d4T, zalcitabine (dideoxycytosine, ddC), nevirapine, lamivudine (epivir, 3TC), abacavir (Ziagen), tenofovir (Viread, TDF), emtricitabine (Emtriva, FTC), saquinavir (Invirase or Fortovase), ritonavir (Norvir), nelfinavir (Viracept), indinavir (Crixivan), amprenavir (Agenerase), fosamprenavir (Lexiva), atazanavir (Reyataz), efavirenz (Sustiva), lopinavir/ritonavir (Kaletra), delavirdine (Rescriptor), and enfurvitide (Fuzeon, T-20). However, none of the available drugs used to combat HIV is completely effective, and treatment frequently gives rise to drug-resistant virus.
Despite the availability of a number of drugs to combat HIV infections, there is a need in the art for additional drugs that inhibit HIV replication, as well as drugs that inhibit viral mutation and the resulting emergence of variant strains that evade host immune response(s) and/or that are resistant to anti-retroviral drugs, which drugs are suitable for treating HIV and other lentiviral infections. The present invention addresses this need by providing methods for identifying agents that inhibit APOBEC3C activity; and by providing therapeutic regimens for treating HIV and other lentivirus infections.
Literature
Harris et al. (2002) Mol. Cell. 10:1247-1253; Yu et al. (Oct. 8, 2004) J. Biol. Chem. Manuscript M408802200; U.S. Patent Publication No. 20040175743; Fitzgibbon et al. (1993) AIDS Res Hum Retoviruses 9:833-838; and Vartanian et al. (1991) J Virol 65:1779-1788.